They always leave it up to the end user. The right wording makes it easier to not get sued.

I don't recall which manufacturer so I'd have to look for it. I've also seen it in online and print documents in industry magazines before.

From what I've seen, taper bearings tend to have a decent life with a fairly wide range of end play, have best life with a little preload and then die quickly with too much preload. In other words, the life of the bearing is the best with a little preload and the life drops off slowly as you move towards too much endplay while it drops off quickly with too much preload.

You can find papers where trucking companies are starting to use specialized equipment to install axle bearings with a preload. It's interesting stuff but basically comes down to them being more reliable when done with precision as opposed to putting the nut tight and then backing it off a little.

I have no idea why the specification is how it is. There are probably good reasons behind it. 0.001" to 0.002" seems like a decent preload target for the sizes of bearing in the transmission. The only reason I could see it not working is if the shaft heats up more than the case and tightens them further, which might be possible on some of the OD gear sets while under significant load. That one bearing is the smallest so it would be the one to watch for issues due to too much preload.